The present invention relates to a plasma-assisted processing apparatus; and, more particularly, the invention relates to a plasma-assisted processing apparatus capable of producing a highly dense, highly uniform plasma under different conditions defined by various parameters, including the types of gases, the pressure of gases and high-frequency power, which are variable in wide ranges, and of satisfactorily processing a workpiece by use of a plasma-assisted process.
Miniaturization of the components of ultralarge-scale integrated circuits (ULSI circuits) has made rapid progress in recent years, and ULSI circuits of minute structure having a minimum feature length on the order of 0.13 xcexcm have been developed. Semiconductor wafers having a large diameter on the order of 300 mm have been used for forming such ULSI circuits thereon. There has been a need for processing apparatuses that are capable of accurately etching workpieces to form DRAMs and flash memories as well as system LSIs, and of processing large-diameter semiconductor wafers.
To meet such a requirement, a plasma-assisted process that is not only capable of highly uniformly processing a large area, but also of having an advanced control characteristic, is necessary. A plasma-assisted processing apparatus must be provided with a plasma-assisted processing unit that is capable of fine processing, and standards for dimensions have become strict. For example, a plasma-assisted etching process must prevent the occurrence of a shape anomaly called a xe2x80x9cnotchxe2x80x9d resulting from the accumulation of positive charges in the bottom of a minute pattern. Negative gases used for etching, such as Cl2, BCl3, SF6 and such, produce negative ions during an etching process. Those negative ions have a function to neutralize positive charges accumulated in the bottom of a minute pattern. Since negative ions are produced more easily at lower electron temperatures, it is desired to realize a plasma having a low electron temperature. Such a plasma of low electron temperature can be produced by a plasma-assisted processing apparatus using high-frequency power at a frequency in the VHF or the UHF band.
In a plasma-assisted processing apparatus, a plasma is produced through the capacitive coupling of an antenna or a counter electrode, when the frequency of high-frequency power applied to the plasma-assisted processing apparatus is 10 MHz or below. The wavelength of the high-frequency power is far smaller than the diameter of the antenna, and any potential distribution is not formed on the antenna. Therefore, a uniform plasma is produced in front of the antenna.
When the frequency of the high-frequency power applied to the plasma-assisted processing apparatus is not lower than the VHF band, the wavelength of the high-frequency power is short, but is long as compared with the diameter of the antenna. Consequently, the uniformity of the plasma produced in front of the antenna is unsatisfactory.
FIG. 12 is a schematic sectional view of a known plasma-assisted processing apparatus using high-frequency power at a frequency in the VHF or the UHF band.
In FIG. 12 the apparatus includes a case 50, a vacuum vessel 51, a processing chamber 52, a workpiece support (electrode) 53 for supporting a workpiece (wafer) 54, a gas supply passage 55, an exhaust passage 56, a first high-frequency power source 57, a high-frequency waveguide 58, a matching device 59, a shield 60, a disk antenna 61, a dielectric material 62, magnetic field creating parts 63, a window 64, a gas-diffusing plate 65 and a second high-frequency power source 66.
The vacuum vessel 51 is disposed in the case 50. The vacuum vessel 51 defines the processing chamber 52. The exhaust passage 56 is connected for evacuation to a lower part of the vacuum vessel 51. The workpiece support 53 supporting the workpiece 54 is located in the processing chamber 52. A large open end of the vacuum vessel 51 is closed hermetically by the window 64 and the gas-diffusing plate 65. The gas supply passage 55 is connected to the gas-diffusing plate 65 to supply gases through the gas-diffusing plate 65 into the processing chamber 52. The disk antenna 61 is placed on the window 64. The disk antenna 61 and the dielectric material 62 are covered with the shield 60. The high-frequency waveguide 58, which penetrates the shield 60, extends through a through hole formed in the case 50 to connect the disk antenna 61 to the external first high-frequency power source 57. The high-frequency waveguide 58 has one end joined to the disk antenna 61 and the other end connected through the matching device 59 to the first high-frequency power source 57. The high-frequency waveguide 58 guides high-frequency power at a frequency in the UHF band (or the VHF band) generated by the first high-frequency power source 57 to the disk antenna 61. The magnetic field creating parts 63 are disposed in the case 50 to create a magnetic field in the processing chamber 52. The second high-frequency power source 66 is connected to the workpiece support 53 to supply high-frequency power at a frequency in the UHF band (or the VHF band) to the workpiece support 53.
When processing the workpiece 54 in the plasma-assisted processing apparatus, gases are supplied through the gas supply passage 55 into the processing chamber 52, the first high-frequency power source 57 applies high-frequency power to the disk antenna 61, the second high-frequency power source 66 applies high-frequency power to the workpiece support 53, and the magnetic field creating parts 63 create a magnetic field in the processing chamber 52. Thus, a plasma is produced in the processing chamber 52. The plasma acts on the surface of the workpiece 54 for plasma-assisted processing.
Since the frequency of the high-frequency power is in the UHF band (or the VHF band), the high-frequency wave carrying the high-frequency power assumes the aspect of an electromagnetic wave. This high-frequency wave propagates only on the boundary region of the plasma and is absorbed. The high-frequency wave is not radiated simply from the disk antenna 61, but also forms a standing wave in a sheath region on the boundary of the plasma and in the high-frequency waveguide 58. The strength distribution of an electric field is dependent on the size and shape of the boundary region of the plasma. To create a high-frequency electric field of a desired strength distribution, such as a flat distribution extending in the direction of the length (diameter) of the workpiece 54, notice must be taken not only of the electric field created in a region under the disk antenna 61, but also of an electric field created around the workpiece 54. This is because, as the high-frequency electric field created around the workpiece 54 tends to enlarge, the high-frequency power is concentrated on the region in which the plasma is produced after the plasma has been produced in the region around the workpiece 54; and, consequently, the density of the plasma around the workpiece 54 increases progressively.
FIGS. 13A to 13D are views and graphs, respectively, which assist in explaining the creation of such a high-frequency electric field. FIG. 13A is a fragmentary sectional view of the plasma-assisted processing apparatus; FIG. 13B is a diagrammatic view showing the strength distribution of an electric field; FIG. 13C is a graph showing the strength distribution of an electric field with respect to a direction along the diameter of the disk antenna 61; and FIG. 13D is a graph showing the variation of power absorption with position with respect to the diameter of the disk antenna 61. In this example, the frequency f of the high-frequency power is 450 MHz, and the window 64 is formed of quartz (specific dielectric constant: 3.5).
In FIG. 13A, there is a sheath region 67, and the other parts like or corresponding to those shown in FIG. 12 are denoted by the same reference characters. In FIG. 13C, the distance (m) from the center of the workpiece 54 is measured on the horizontal axis, and the ratio of the electric field strength Eedge at an optional part of the workpiece 54 to the electric field strength Ecenter at a central part of the workpiece 54 is measured on the vertical axis. In FIG. 13D, the distance (m) from the center of the workpiece 54 is measured on the horizontal axis, and the power of an electromagnetic wave absorbed by the plasma (absorbed power) is measured on the vertical axis.
As shown in FIG. 13B, a high-frequency wave having a frequency in the UHF band propagates through the window 64 and the sheath region (boundary region of the plasma) 67. As shown in FIG. 13C, a strength distribution curve, indicating the strength distribution of the electric field created right below the window 64, has a node at a part corresponding to the distance 110 mm from the center of the workpiece 54 (TM01 mode), and a part of the electric field around a peripheral part of the workpiece 54 has an electric field strength Eedge. The high-frequency wave having a frequency in the UHF band is concentrated on a part where the plasma density increases, further enhancing the concentration of the high-frequency wave of the frequency in the UHF band on the same part. Consequently, as shown in FIG. 13D, the absorbed power of the high-frequency wave and the plasma density distribution change when the high-frequency power (density) is changed.
In a known plasma-assisted processing apparatus using a high-frequency wave having a frequency in the VHF or the UHF band, the uniformity of the plasma density distribution in front of the antenna is disturbed and the plasma density distribution changes when the high-frequency power (density) is changed.
There are some known plasma-assisted processing apparatuses; using a high-frequency wave having a frequency in the VHF or the UHF band, that are capable of producing an improved plasma. A first known plasma-assisted processing apparatus, as disclosed in Japanese Patent Laid-open No. 2000-195843, is provided with a disk-shaped counter electrode disposed opposite to a wafer, i.e., a workpiece, with a dielectric material being disposed between the counter electrode and the wafer. A second known plasma-assisted processing apparatus, as disclosed in Japanese Patent Laid-open No. 7-307200, is provided with a radial antenna structure for radiating a high-frequency wave, formed by alternately arranging a plurality of antenna elements radially extending from the center of the antenna and a plurality of antenna elements extending from the periphery toward the center of the antenna. A third known plasma-assisted processing apparatus, as disclosed in Japanese Patent Laid-open No. 10-12396, is provided with an antenna structure including an inner antenna conductor and an outer antenna conductor, having a length different from the inner antenna, disposed at different levels, respectively, to form a resonance structure for producing a uniform plasma. A fourth known plasma-assisted processing apparatus, as disclosed in Japanese Patent laid-open no. 2000-195843, is provided with a disk-shaped electrode (antenna) disposed opposite to a wafer, i.e., a workpiece, and provided with an annular groove as a plasma trap to produce a uniform plasma and to control the plasma density distribution on the wafer.
The first known plasma-assisted processing apparatus is intended to moderate the potential distribution of the high-frequency wave on the counter electrode by disposing the dielectric material between the antenna and the counter electrode. However, the production of a plasma by an electromagnetic wave that propagates along the surface of the counter electrode is dominant because the plasma is produced by capacitive coupling dependent on the potential distribution of the high-frequency wave, and the effect on the moderation of the potential distribution of the high-frequency wave on the counter electrode is comparatively unsatisfactory.
The second known plasma-assisted processing apparatus uses a radial antenna structure, in which intervals between the antenna elements increase toward the periphery of the radial antenna structure. Therefore, the electric field strength in a region around the peripheral part of the radial antenna structure is low, and boundary conditions for the electromagnetic wave in a region in which the antennal element exists and in a region in which any antenna element does not exist are different. Therefore, the electric field strength is not fixed with respect to the circumferential direction on the radial antenna structure.
In the third known plasma-assisted processing apparatus, an intense electromagnetic wave radiated by the antenna propagates through the sheath region when a desired resonance structure is formed. Therefore, the pattern of antenna radiation is different from the pattern of the electric field in the sheath region, and, hence, the plasma density is not necessarily distributed in a uniform manner.
In the fourth known plasma-assisted processing apparatus, the plasma trap formed on the counter electrode is in a plasma-producing region. Therefore, an electromagnetic wave radiated from the counter electrode is enhanced by the plasma trap, the plasma density around the plasma trap increases, the high-density plasma produced in the region diffuses into a region around the workpiece, and a uniform plasma, which is more uniform than a plasma produced by a plasma-assisted processing apparatus not provided with any plasma trap, is produced around the workpiece. However, since the plasma flows into the annular groove serving as the plasma trap, it is difficult to produce a still more uniform plasma.
Those known plasma-assisted processing apparatuses using a high-frequency wave having a frequency in the VHF or the UHF band have difficulty in producing a uniform plasma in a region in which the workpiece is placed, and they take nothing into consideration to prevent the variation of the density of the plasma produced around the workpiece dependent on the variation of the process parameters.
The present invention has been made in view of the above-mentioned a technical background, and it is therefore an object of the present invention to provide a plasma-assisted processing apparatus that is capable of producing a highly uniform, high-density plasma around the entire workpiece by using a high-frequency wave having a frequency in the VHF or the UHF band, regardless of a variation of the process parameters.
According to the present invention, a plasma-assisted processing apparatus includes: a vacuum vessel defining a processing chamber; a gas supply line for carrying gases into the processing chamber; a workpiece support for supporting a workpiece, disposed in the processing chamber and serving as an electrode; a disk antenna for radiating a high-frequency wave having a frequency in the VHF or the UHF band into the processing chamber; a high-frequency waveguide for guiding a high-frequency wave to the disk antenna; and a window of a dielectric material isolating the disk antenna from the processing chamber; wherein a conductive ring is disposed between the disk antenna and the window, such that its end surface is in contact with a peripheral part of the disk antenna.
The conductive ring is disposed with its end surface in contact with the peripheral part of the disk antenna to generate a standing wave in a space surrounded by the conductive annular ring. Thus, the strength of a part of an electric field in the space surrounded by the conductive ring is enhanced, and the strength of a part of the electric field around the conductive ring decreases relatively. Therefore, the variation of power absorbed by the plasma, i.e., the variation of the plasma density distribution, can be suppressed even if the high-frequency power (density) varies.
A second window made of a dielectric material may be superposed on the first window, and the first window and the second window may be formed of different dielectric materials respectively having different dielectric constants, respectively. Use of such a technique enhances the standing wave in the boundary region of the plasma through the enhancement of the high-frequency standing wave in a region right under the disk antenna, and the function of the structure can be enhanced.
An antenna height adjusting means that is capable of moving the disk antenna to adjust the distance between the disk antenna and the first window, or to adjust the distance between the disk antenna and the first window and the distance between the disk antenna and the second window, may be connected to the disk antenna. Thus, the position of a node in the high-frequency standing wave right under the disk antenna can be moved along the diameter of the disk antenna, whereby the plasma density distribution can be optionally adjusted to a plasma density distribution compatible with gases and a film to be processed.
The conductive ring may be formed in an inside diameter in the range of an integral multiple of half the wavelength of the high-frequency wave propagating through the conductive ring xc2x110%. Thus, the high-frequency standing wave right under the disk antenna can be enhanced, and, hence, a plasma can be easily produced right under the disk antenna.
A conductive member having the shape of a rod or a cylinder having a height equal to that of the conductive ring may be disposed in a central region of a space surrounded by the conductive ring and corresponding to a central part of the disk antenna. Thus, the strength of a part of the high-frequency electric field corresponding to the central part of the disk antenna can be enhanced, whereby the processing speed at which a central part of the workpiece is processed can be increased.
A dielectric ring or cylinder having a height nearly equal to that of the conductive ring may be disposed in central region of a space surrounded by the conductive ring and corresponding to a central part of the window. Thus, the reduction of the strength of a part of the high-frequency electric field corresponding to a central part of the disk antenna can be avoided, whereby a uniform electric field can be created around the central part of the disk antenna.
The disk antenna and the high-frequency waveguide may be formed in dimensions meeting an inequality: a/Rdxe2x89xa60.4 or a/Rdxe2x89xa60.6, where a is the radius of the disk antenna, and Rd is the effective radius of the high-frequency waveguide. Thus, the strength of a part of the high-frequency electric field around the disk antenna can be reduced, and the density of a part of the plasma around the disk antenna can be reduced, whereby a highly uniform, high-density plasma can be stably produced around the entire workpiece.